In the related art, a seat has been adopted having a seat back provided with side portions onto which a pair of right and left side frames are provided, the side frames being provided with weakened portions, respectively (see, for instance, Japanese Patent No. 4000731). The weakened portions include inclined portions each bent in a triangular shape in cross section. When a load is imparted to the seat at a vehicle front or a vehicle rear, the inclined portions are caused to bent and deform thereby absorbing impact energy.
With the seat frame described in Japanese Patent No. 4000731, however, the inclined portions are located on the side frames at front ends thereof in the vicinity of a surface of the seat back. Accordingly, as a vehicle occupant sits on the seat, the inclined portions project toward the vehicle occupant, causing an issue of the vehicle occupant having a sense of incongruity with degradation in sitting comfort.